1. Field of the Invention
The present invention generally relates to coating apparatuses of the type used to deposit ceramic and metallic coatings. More particularly, this invention relates to a physical vapor deposition (PVD) apparatus and process for depositing a coating whose composition contains elements with relatively low and relatively high vapor pressures, such as a nickel aluminide alloyed with zirconium, hafnium, yttrium and/or cerium.
2. Description of the Related Art
Components within the turbine, combustor and augmentor sections of gas turbine engines are susceptible to oxidation and hot corrosion attack, in addition to high temperatures that can decrease their mechanical properties. Consequently, these components are often protected by an environmental coating alone or in combination with an outer thermal barrier coating (TBC), which in the latter case is termed a TBC system.
Diffusion coatings, such as diffusion aluminides and particularly platinum aluminides (PtAl), and overlay coatings, particularly MCrAlX alloys (where M is iron, cobalt and/or nickel, and X is an active element such as yttrium or another rare earth or reactive element), are widely used as environmental coatings for gas turbine engine components. Ceramic materials stick as zirconia (ZrO2) partially or fully stabilized by yttria (Y2O2), magnesia (MgO) or other oxides, are widely used as TBC materials. Used in combination with TBC, diffusion aluminide and MCrAlX overlay coatings serve as a bond coat to adher, the TEC to the underlying substrate. The aluminum content of those bond coat materials provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) protects the bond coat from oxidation and hot corrosion, and chemically bonds the TBC to the bond coat.
Suitable processes for depositing MCrAlX coatings include thermal spraying such as plasma spraying and high velocity oxyfuel (HVOF) proceasca, and physical vapor deposition (PVD) processes such as eLectron beam physical vapor deposition (EBPVD), magnetron sputtering, cathodic arc, ion plasma, and pulsed laser deposinon (PLD). PVD processes require the presence of a coaling source material made essentially of the coating composition desired, and means for creating a vapor of the coating source material in the presence of a substrate that will accept the coating. FIG. 1 schematically represents a portion of an EBPVD coating apparatus 20, including a coating chamber 22 in which a component 30 is suspended for coating. An overlay coating 32 is represented as being deposited on the component 30 by melting and vaporizing an ingot 10 of the desired costing material with an electron beam 26 produced by an electron beam gun 28. The intensity of the beam 26 is sufficient to produce a stream 34 of vapor that condenses on the component 30 to form the overlay coating 32. As shown, the vapor stream 34 evaporates from a pool 14 of molten coating material contained within a reservoir formed by a wall 18 of a crucible 12 that surrounds the upper end of the ingot 10. Water or another suitable cooling medium flows through cooling passages 16 defined within the crucible 12 to maintain the crucible 12 at an acceptable temperature. As it is gradually consumed by the deposition process, the ingot 10 is incrementaily fed into the chamber 22 through an airlock 24.
More recently, overlay coatings (i.e., not a diffusion) of beta-phase nickel aluminide (β NiAl) intermetallic have been proposed as environmental and bond coat materials. The NiAl beta phase exists for nickel-aluminum compositions of about 30 to about 60 atomic percent aluminum, the balance of the nickel-aluminum composition being nickel. Notable examples of beta-phase NiAl coating materials include commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., which discloses a NiAl overlay bond coat optionally containing one or more active elements, such as yttrium, cerium, zirconium or hafnium, and commonly-assigned U.S. Pat. No. 6,291,084 to Darolia et al., which discloses a NiAl overlay coating material containing chromium and zirconium. Commonly-assigned U.S. Pat. Nos. 6,153,313 and 6,255,001 to Rigney et al. and Darolia, respectively, also disclose beta-phase NiAl bond coat and environmental coating materials. The beta-phase NiAl alloy disclosed by Rigney et al. contains chromium, hafnium and/or titanium, and optionally tantalum, silicon, gallium, zirconium, calcium, iron and/or yttrium, while Darolia's beta-phase NiAl alloy contains zirconium. The beta-phase NiAl alloys of Nagaraj et al., Darolia et al., Rigney et al., and Darolia have been shown to improve the adhesion of a ceramic TBC layer, thereby increasing the service life of the TBC system.
MCrAlY-type coatings have been deposited using PVD and thermal spray processes interchangeably to achieve acceptable levels of environmental protection. Although no advantages have been found for either deposition technique in terms of environmental performance, PVD processes have often been preferred for producing smooth as-coated surface finishes and coating thickness uniformity. In contrast, beta-NiAl coatings have been found to exhibit significantly improved spallation resistance and environmental properties if deposited by a PVD process. However, difficulties have been encountered when attempting to deposit beta-NiAl coatings alloyed with zirconium, hafnium, yttrium, cerium and/or another reactive element, as required by the NiAl compositions of Nagaraj et al., Darolia et al., Rigney et al., and Darolia. For example, when attempting to evaporate an ingot of NiAl+Zr or NiAl+Cr+Zr, deposition rates are low and the chemistry of the coating has been difficult to control.